CBSE Class 9, Chapter-14, NCERT Science

Natural Resources

CBSE Guide - CBSE Sample Questions with Solutions

Question.1: What is ‘Water Cycle’ ? Explain the process of water
cycle.

Solution: The process by which water
continually changes its form and circulates between oceans, atmosphere and land
is called ‘Water cycle’.

When temperature
increases water evaporates from the surface of water bodies and changes into
water vapour. When these water vapours reach to a certain height they get
cooled and turn into very tiny water droplets. These water droplets combine
together to form clouds. When they become heavy they come down to the surface
of the earth in the form of rain or snow. This is the process of circulation of
water known as ‘Water Cycle’.

Question.2: Write a short note on ‘Nitrogen Fixation’.

Solution: It is a combination of natural and
industrial processes by virtue of which the free atmospheric nitrogen is
converted into nitrogen compounds such as - ammonia, nitrates, nitrites etc.
that is essential for plant growth and is also used by chemical industries.

Nitrogen is also fixed
as nitric oxide by physical processes e.g. Lightning and Ultraviolet (UV) Rays.
But more significantly nitrogen is fixed as ammonia, nitrites and nitrates by
soil micro-organisms like - bacterium Rhizobium.
Rhizobium lives in the root nodules
of leguminous plants (pulses, beans and peas) with which it has a symbiotic
relationship. Due to nitrogen fixation the fertility of soil is increased.

Question.3: Explain the ‘Nitrogen Cycle’. or,

Write a short note on ‘Nitrogen Cycle’.

Solution:

Nitrogen Cycle

It is a natural cyclic
process in the course of which atmospheric nitrogen enters the soil and becomes
a part of living organisms before returning to the atmosphere. Nitrogen an
essential part of the amino acids, proteins and nucleic acids is a basic
element of life. Although 78% by volume of the atmosphere is nitrogen gas, but
this gaseous nitrogen must be converted to some usable forms like nitrates and
nitrites before it can be consumed by living organisms. This is accompanied
through the nitrogen cycle. Certain bacteria found in the roots of legumes and
blue green algae present in the soil fix nitrogen from atmosphere and convert
it into inorganic nitrogen compounds. Such compounds are directly taken up by
plants through their root-system. The nitrogen then passes through the
food-chain from plants to herbivores and carnivores. When plants and animals
die, bacteria and fungi present in the soil convert the nitrogenous wastes back
into nitrogenous compounds (nitrates and nitrites) to be used by plants again.
Certain other bacteria convert some part of these nitrogenous wastes to free
nitrogen through ‘Denitrification’, which goes back into the atmosphere.

As a result, the
percentage of nitrogen in the atmosphere remains more or less constant. Thus,
there is a nitrogen cycle in nature in which nitrogen passes from its elemental
form in the atmosphere into simple molecules in the soil and water, which get
converted to more complex molecules in living beings and back again to the
simple nitrogen molecule in the atmosphere. [Please
refer to and give the figure of Nitrogen Cycle given in the Class IX NCERT
science textbook].

Question.4: Discuss the consequences of the increase in the
concentration of Carbon Dioxide and other Green House gases in the atmosphere.

Solution: An increase in the content of Carbon
Dioxide (CO2) or any of other green house gases in the atmosphere
would cause more heat to be retrained by the atmosphere leading to an increase
in the earth’s temperature. This global warming is dangerous as it would cause
melting of glacier and thereby flooding the coastal plains and also an
ecological imbalance.

Question.5: What are the causes of Soil Erosion ?

Solution:

Causes of Soil Erosion

The soil that we see
today has been created over a very long period of time. some of the main causes
or factors of soil erosion are as follows:

1. Strong wind - The soil which
is devoid of vegetation or loose, is eroded when exposed to strong winds. The
wind carries away the soil particles with it and exposes the bed rock.

2. Heavy rain - When rain falls
on the unprotected top soil, the loose soil particles are washed away down into
the streams.

3. Human activities - Human
activities such as various constructions, expansions of urban areas,
industrialization etc. lead to deforestation leading to exposure of top soil to
the various agencies of soil erosion.

4. Overgrazing - Overgrazing by
our domestic animals not only destroys the biodiversity but leads to soil
erosion.

5. Floods - Frequent flooding
of rivers is another cause of soil erosion.

6. Improper farming and suspended
cultivation - If the tilled (ploughed / loosened) soil is left uncultivated
for long time for whatever reason, then it causes to soil erosion.

7. Deforestation -
Deforestation due to so many reasons is a major cause for soil erosion.